This application is related to audio speaker systems in portable computers.
Background: Computer-Based Audio
The production of quality sound with computers has advanced significantly in recent years. Early personal computers typically had nothing more than a single, small speaker used to produce a beep or series of single-frequency beeps to indicate system status upon startup. However, consumers demanded more. With the significant advances in circuit miniaturization, improved sound quality electronics has been available for desktop systems for some time, and is now moving into the portable computer systems arena.
Today, there is a multimedia explosion that is adding a significant new dimension to the way people use computers. One of the biggest advantages is 16-bit, CD-quality sound. With the right speakers, this sound can be as impressive as the sound on a normal consumer""s stereo system. More and more, computer applications include the capability to use and capture the full impact of 16-bit sound for multimedia. MIDI musical compositions become more inspiring, games more suspenseful, software more dramatic, and radio and television programs (obtained through the computer) can equal the quality of conventional reception systems.
In addition, computer and audio technologies are revolutionizing business. For example, in video conferencing, voices are reproduced with a natural quality that makes them seem as if the speaker were in the room. Providing a high-quality audio background during presentations greatly enhances their effectiveness, and portable computers provide the greatest opportunity for meeting this need. Furthermore, the work environment is becoming ever more mobile with employees traveling more, and perhaps even telecommuting (working from home). Use of increased travel time translates to work environments with higher ambient noise, such as airports, trains, or mass transit systems. Another factor in this trend is the user""s need for more effective methods for communicating with potential customers, or even with the office when away on business.
Background: Spatial Impression
The design of sound reproduction systems is not only based on considerations of electrical and acoustical engineering and physics, but also requires knowledge of psychoacoustics, i.e. how sound is perceived by listeners.
One of the parameters of psychoacoustics is spatial impression. When a sound is generated in a room, the listener will first hear the sound via the direct path from the source. Shortly thereafter, the listener will hear the reflections of the sound off surfaces such as walls or ceilings. Human listeners will assess the size of the space they are in by listening to laterally reflected sound which accompanies a sound signal. Thus, in a loudspeaker system it is desirable to have some sound transmission paths which reach the ears of the listener with a certain amount of delay (e.g. 10-60 milliseconds) as compared with the direct transmission path. (This will give the impression of a spacious listening room by broadening the soundstage, and also by giving the illusion of pushing the sound beyond the physical location of the speakers, even if the listener is listening in a room which is very small.) Moreover, another peculiarity of this psychoacoustic parameter is that the low frequencies (below 500 Hz) dominate spatial impression. A more roomy spatial impression is welcome to many listeners. However, it is not easy to do this with a small speaker system, and particularly not with a small portable computer""s speaker system.
To increase spatial impression, the ratio of laterally reflected energy to directly transmitted energy can be increased: the higher this ratio, the greater the spatial impression. (An extreme case of this is found in a symphony concert hall, where there is almost no direct sound.)
Whether the user is video-conferencing, playing a game, or just working with music in the background, spatial impression plays an important role in the computing experience. That role is growing ever more important as multimedia makes its way into even the most uncompromising of business applications.
Further background regarding spatial impression can be found in: J. Blauert, SPATIAL HEARING (2.ed. 1996); and in M. Barron, xe2x80x9cEffects of Early Reflections on Subjective Acoustic Quality in Concert Hallsxe2x80x9d (thesis, University of Southampton, 1974); both of which are hereby incorporated by reference.
Background: Stereophonic and Surround Sound
Since its introduction in the 1950""s, stereo has been regarded as an essential minimum requirement of quality sound reproduction. Stereo can convey a traditional soundstage in which the sound comes from the front, such as when one attends a play or concert. However, even stereo has shortcomings when required to convey the ambiance where the sound is heard from all around the listener. Stereo""s lack of spatiality undermines sonic realism in, for example, a game where aircraft fly overhead from front to back, or footsteps come from off to the side. For this reason various xe2x80x9csurround soundxe2x80x9d schemes have been used, to provide at least some speaker output behind the listeners"" positions.
Background: Portable Computer Audio Limitations
Two driving constraints on the development of portable computers are volume and power consumption. The form factor of portable computers cannot be expanded, so that volume is a key resource. Additional functions are continually sought to be added, and many of these place new demands on available volume. Moreover, as larger display sizes become available they require more area in the lid, and hence consume more volume. Similarly, users continually demand more battery lifetime, which implies a continuous struggle to minimize power consumption.
These two constraints bind especially tightly in the case of speakers: the low-frequency power-efficiency of a speaker is directly affected by its volume. Since low-frequency response is a significant limitation of portable speaker systems, this is a difficult constraint. Some attempts have been made to use external speakers with portable computers, but this is cumbersome.
As of 1997, the internal sound systems typically sold with portable computers produce marginal sound quality at best. Most multimedia computers sold today include a stereo sound card which is capable of reasonably good sound reproduction. However, the internal speakers are typically small and produce tinny sound with inadequate bass. As a result, millions of multimedia computer users suffer very poor sound fidelity from their computers.
However, poor sound quality in portable computers is undesirable. The experience of a game or multimedia presentation, with sound and video animation, can be astonishingly real when coupled with quality audio. Quality sound can make a significant difference in the enjoyment the user can get from a multimedia computer. Music while working is more enjoyable, training and xe2x80x9cedutainmentxe2x80x9d titles are richer, and games become much more realistic and exciting.
Background: Subwoofers
One of the challenges in high-fidelity audio reproduction is low-frequency reproduction. The extreme lower limit of human hearing is around 20 Hz (about an octave below the bottom note on a bass guitar) for very loud sounds. However, human hearing becomes less sensitive at low frequencies, so the threshold of audibility for such low sounds is relatively high. While recorded music seldom uses frequencies below 30 Hz, frequencies down to about 30 Hz do appear in orchestral recordings. Moreover, frequencies down to 30 Hz or even below may be used for special effects. This is a problem, because the acoustic impedance of many conventional speaker configurations becomes very small below 50 Hz or so, and this means that such speaker systems are inefficient at turning electrical signals into sound waves.
One conventional audio engineering technique for enhancing low-frequency reproduction is subwoofers. A conventional subwoofer is an additional speaker which is designed to produce deep bass (and only bass). Thus the design specifications for a subwoofer typically cover only a limited range of frequencies (typically 3 octaves or less), so the subwoofer can be optimized for efficiency and power-handling in this range. Low-pass filtering is typically used to prevent the subwoofer from exposure to midrange signals, so midrange distortion (which can be caused e.g. by cone breakup with large cone diameters) is not a concern. Thus subwoofers in component systems commonly use large cone diameters, such as 12 inches or more. The cabinets of conventional subwoofers are often designed with tuned-port resonances which will optimize efficiency at low frequencies.
Typically only one subwoofer is used. The human ear is virtually unable to detect the origin of sufficiently deep bass sounds, so having two subwoofers is of no benefit from the standpoint of stereo separation. (For example, a bass guitar""s low E has a wavelength of about 27 feet, and human ears are too close together for directional hearing at such wavelengths.) Therefore, it is not necessary to have the subwoofer in a line-of-sight or even pointing toward the listener""s ears. When using a subwoofer with a pair of satellite speakers (producing a stereo effect), the listener""s ears are fooled into xe2x80x9chearingxe2x80x9d all of the sound, including the bass, coming from the satellites. In component systems, a good quality subwoofer operates in frequency down to 35 Hz or lower. Over 150 Hz, it will start interfering with the stereo effect of the satellite speakers.
Smaller xe2x80x9csubwoofersxe2x80x9d have also been used in volume-limited audio systems, such as automotive systems. Such xe2x80x9csubwoofersxe2x80x9d are not of the same dimensions as those in component systems, but still serve to extend the range of the total speaker system. For example, in a portable computer system which has small primary left and right speakers whose response rolls off below 300 Hz or so, a subwoofer in an expansion base would typically have an upper frequency of 500 Hz or so.
Portable Computer Expansion Base with Enhancement Speaker
The present application discloses the use of a portable computer expansion base with an added low-frequency-capable speaker which is not merely a subwoofer. The response of this added speaker not only provides subwoofer effects, but also is extended above the subwoofer range to provide other functions. Specifically, the response is extended high enough to provide directionality, and the direction of acoustic emission is primarily toward the rear of the unit, away from the listener. This rearward emission provides multiple benefits, at frequencies above the subwoofer range, which in various embodiments may include some or all of the following:
enhanced spatial impression;
greater total midrange power;
greater front-back depth of the acoustic image; and
some enhanced surround-sound impression (depending on the room environment), due to acoustic reflections from the rear-firing speaker.